1. Field of the Invention
The present invention relates to methods for fabricating composite structures such as composite panels and structural members that include a fabric impregnated with resin. Even more particularly, the present invention relates to an improved method of producing composite components wherein fibrous reinforcements are assembled into a desired configuration and upon a tool. A vacuum is applied to the reinforcements which consolidate the reinforcement and evacuates air therefrom. A resin is applied to the reinforcement by using a flow path therethrough having entry and exit ports. A vacuum is used to transmit the resin through the reinforcement between entry and exit ports. The resin flows into the reinforcement due to the pressure differential between the evacuated reinforcement and the hydrostatic pressure on the resin and also due to the capillary effect of the reinforcement.
2. General Background
Composite fabrication processes can be very expensive because of the labor required to form the laminate in the desired configuration and the need for autoclaves to create the heat and pressure necessary to consolidate and cure the resin and the reinforcement. Additionally, very expensive presses have been used to compress and cure resins and fibrous reinforcement material and to configure those materials into a desired shape in the formation of a composite structure. Advanced composite structures such as are used in the aerospace industry require very particular types of reinforcements and of resins which typically dictate the need for heat and pressure in order to form a "pre-preg" into a consolidated cured laminate. A "pre-preg" is a material which consists of a fibrous reinforcement impregnated with a resin that is partially cured to a state in which it can be handled. The use of pre-preg material is expensive due to specific construction steps involved in making pre-pregs. They typically must be specially packaged, stored, and have a definite shelf life. The use of pre-pregs in the formation of composite structures is known in the art and is generally regarded as an expensive process.
The present invention provides an improved method and apparatus for composite fabrication which dramatically reduces overall costs of composite structures. This cost reduction is achieved without sacrificing mechanical or structural performance. The method and apparatus of the present invention are capable of producing complex aerospace quality components of a wide variety of scale and configuration. Possible applications of this technology include, for example, the space shuttle external tank intertank member, payload fairings for launch vehicles, intertanks for launch systems, ailerons, fairings, spoilers, stabilizers, wing skins for aircraft, sonar transmissible fairings for underwater vessels, radomes, and the like. The present invention provides a method for producing composite components that includes the step of first assembling a number of dry fibrous reinforcements, or preform, into a desired configuration such as by stacking a plurality of fibrous reinforcements onto a forming tool. A vacuum is then applied to the reinforcement in order to evacuate air therefrom and to consolidate the reinforcement.
A resin is applied to the reinforcement by using a flow path therethrough having an entry and an exit point. The vacuum is used to transmit the resin through the reinforcement and between the entry and exit points. The resin flows into the reinforcement due to the pressure differential between the evacuated reinforcement and the hydrostatic pressure on the resin as well as the capillary effect of the reinforcement itself.
The resin can be heated prior to introduction to the reinforcement if desired. This is particularly helpful in dealing with certain exotic resins used in the aerospace industry in which room temperature viscosity inhibits consistent flow. The fibrous reinforcements can be heated prior to and during the introduction of resin to the reinforcement as well.
The reinforcement infiltration process of the present invention provides a means of producing composite components using dry (impregnated) fibrous reinforcements and liquid resin in a non-autoclave process which does not require expensive closed mold tooling or injection machines. The fibrous reinforcements (which can be plies of fabric, mat, or stitched preforms) are first assembled into a desired configuration such as upon a tool or other shaped support.
The resin containment and compaction system is then assembled to the reinforcement and a vacuum applied. During this procedure, the tool, reinforcement and resin can be heated if desired. The heating step lowers the viscosity of the resin and allows the resin to flow more freely through the reinforcement.
The reinforcement is evacuated by using a vacuum source and then the resin is connected to the resin containment and compaction system. The resin is then allowed to flow into the peripheral reinforcement and from that point flows into the reinforcement due to the pressure differential between the evacuated reinforcement and the hydrostatic pressure on the resin as well as the capillary effect of the reinforcement.
The resin is allowed to flow into the reinforcement until the reinforcement is fully impregnated, and any remaining air in the reinforcement is removed during such flow of resin therethrough. Excess resin is vented out of the vacuum attachment in the center portion of the apparatus and then contained in a resin trap. The temperature of the reinforcement is then raised to gel and cure the resin.
The method and apparatus of the present invention has been used to produce laminates with similar quality and performance to traditional autoclave processing. The process can be used to fabricate flat panels as well as complex geometries as well as transitioning a skin/stringer panel into a metal flange for example.
When compared to hand lay-up/autoclave cure, the present invention has the same versatility to produce the complex geometries, but offers significant reductions in cost. These result from a reduction in material cost by fifty to seventy five percent (50-75%) due to the fact that resin and fiber are used in raw form, as opposed to being combined into a "pre preg" (a pre-impregnated, partially cured sheet material used for hand lay-up). Labor costs are also reduced because parts can be fabricated using woven or stitched preforms. A preform is a type of reinforcement in which the fibers have already been oriented and positioned into its final orientation by automated machinery.
With the method of the present invention, a preform is placed into the mold, compaction is applied, resin is introduced, and the part is cured. With hand lay-up, each ply must be form fitted to the mold and properly oriented individually. Often the plies must be periodically "de-bulked", or compacted with a vacuum bag to reduce bulk, remove entrapped air and allow proper conformance of the pre preg to the tool.
Another advantage of the present invention over hand lay-up is that the method of the present invention does not require an autoclave (an enclosed vessel which applies high pressure and heat for laminate cure). Autoclaves significantly drive up fabrication costs and limit size of parts which can be fabricated. The method of the present invention makes use of ovens or integrally heated tooling, both of which are orders of magnitude less expensive to procure than autoclaves, especially for very large parts.
When compared to traditional resin transfer molding (RTM), the present invention offers much less costly tooling, much larger size capability, higher fiber volumes, and less expensive processing equipment. Resin transfer molding is similar to resin infiltration in that resin is injected into a fibrous preform; however, resin transfer molding requires an expensive closed mold and high pressure resin injection equipment. The closed mold must define all surfaces of the part being fabricated, must maintain close tolerances and fabricated to withstand the high pressures required for resin injection. Such tools are very expensive to produce and maintain. The present invention requires a tool with only one surface; the other surfaces are defined by the conformable compaction system.
The tooling system for the present invention provides great advantages in fabricating parts, including for example, parts with smooth outside surfaces and/or with stiffeners on the inside. For such a part, a tool with a smooth surface could be used with the method of the present invention, and removable mandrels or parasitic tooling could be used to shape the stiffeners.
A resin transfer molding tool for such a part would be very expensive because the exact dimensions and configuration for each stiffener would be required in one half of the tool.
The use of one sided tooling reduces costs of preforms and allows for higher fiber volumes. With traditional resin transfer molding, the preform must be tightly woven to very close tolerances such that it will fit inside a closed space. Because it is very difficult to weave a dense preform to the final dimensions of a part, the percent fiber volume of a given part is limited by the thickness of preform.
With the present invention, the preform is compacted after the conformable compaction system is in place, which allows for higher fiber volume parts. Another advantage of the method of the present invention over resin transfer molding is its size capability. Resin transfer molding is limited first by tooling; large, closed molds tools are prohibitively expensive, especially for any complex geometry. Secondly, resin transfer molding is size limited due to the injection pressures required to impregnate large parts. Large parts may force injection pressures to exceed the capability of available injection equipment or to exceed the pressure required to separate a closed mold.